Liquid crystal display devices using nematic liquid crystals may be classified roughly into those operating by the dynamic scattering mode (DSM) and those operating by the field-effect mode (FEM). FEM liquid crystals may in turn be divided into those showing the DAP effect and those operating in the twisted nematic mode, according to the way of orientation of the liquid crystal molecules. The DAP effect mode includes those nematic liquid crystals having negative dielectric anisotropy and homeotropic alignment wherein the molecular axes are aligned perpendicularly to the plates. The twisted nematic mode crystals includes those having positive dielectric anisotropy and homogeneous alignment wherein the molecular axes of the liquid crystal molecules proximate the plates are aligned in parallel with the plates. Further, where the plate is so treated as to have a preferred direction as by rubbing with cotton or other mild abrasives and the plates are mounted so that the rubbing directions are at an angle to each other, then the liquid crystal molecules at the opposed surfaces make an angle with each other and liquid crystal molecules between the plates form a helix with the molecules at the ends of the helix corresponding in direction to the rubbing direction of the plates
The rubbing direction may be termed an orientation of the inner surface of the plates. It is now believed that minute grooves are formed, all of which are essentially parallel when the plate is unidirectionally rubbed. Liquid crystal molecules proximate the surface of the plates fall into these grooves with their molecular axes parallel to the grooves. As aforenoted, when a pair of plates are opposed to each other and spaced apart so that a cell may be formed which can contain liquid crystal molecules, a helical structure results. This helical structure is stable only in the absence of an electric or magnetic field. If a sufficiently strong electric field is applied the spiral structure is broken down, this being due to the dipole moments of the molecules causing them to orient themselves parallel to the field. As a result of the helical structure, a cell containing liquid crystals such as are under discussion here can rotate the plane of polarization of linearly polarized light through an angle corresponding to the twist angle between the plates. When a sufficiently strong electric field is applied between the plates, the optical activity disappears.
As aforenoted, the liquid crystal molecules adjacent a surface follow the orientation of that surface. However, in lining up within the grooves, the molecules within a groove or in adjacent grooves may be either parallel to each other or anti-parallel to each other. Accordingly, the terms normal direction (the "normal" direction being selected arbitrarily) and the reverse direction may be used. Since there are two concurrent directions at each of the inner surfaces, there are four combinations in all. In the conventional twisted nematic mode liquid crystal display device, although not all four combinations may be produced as evidenced by the twist direction of helices formed between the plates, there are, nevertheless, a plurality of combinations formed. The different combinations are observed as domains on the display surface. The alignment of the liquid crystal molecules is therefore different in each domain. The practical result of this phenomenon is that the contrast observed on activation of a display varies with the direction of observation. Also, the intensity of the display will be a function of the point from which the display is viewed. It would be desirable to eliminate the difference in contrast resulting from the phenomenon described; in order to do this, it would be necessary to align the liquid crystal molecules so that the liquid crystal layer is, in effect, monocrystalline. To put this in another way, the twist direction of the great majority, and if possible, all, of the helices between the plates should be the same.